1. Field of the Invention
The present invention relates to an improved electrodeionization apparatus capable of preventing scale deposition of hardness components.
2. Description of the Related Art
Ion exchange resins have been used to produce deionized water. These ion exchange resins generally require chemical regeneration. To avoid this, a deionizing function of the ion exchange resins and an electrodialysis function of ion exchange membranes are combined in an electrodeionization (EDI) apparatus to obtain high-purity deionized water without chemical regeneration.
A typical EDI apparatus comprises a series of desalination chambers each having a cation exchange membrane on one side, and an anion exchange membrane on the other side. The space between the two membranes is filled with cation and anion exchange resins. Concentrate chambers are provided on both sides of each desalination chamber in which concentrate water flows. Water to be treated is passed through the desalination chambers while a direct current is applied to the desalination chambers across the ion exchange membranes. As a result, impurity ions are electrically moved from the water to be treated, via ion exchange resins, to the concentrate water flowing in the concentrate chambers outside of the ion exchange membranes. Thereby, deionized water is produced in the desalination chambers and impurity ions are concentrated in the concentrate water.
The concentrate water is once discharged from the apparatus, but is not discarded. More specifically, the concentrate water is reused to improve the water utilization rate (recovery rate) of the EDI apparatus. That is, the concentrate water discharged from the EDI apparatus is returned to the inlet side of the concentrate chamber, while the water to be treated is supplied to the EDI apparatus. A portion of the circulating concentrate water is discharged out from the apparatus. By circulating and reusing the concentrate water in this manner, the water utilization rate is improved and a reasonable ion concentration in the concentrated water is maintained. By employing this circulating method of concentrate water, the ion concentration within the concentrate water increases and this in turn leads to higher electrical conductivity of the concentrate water. Thus, electricity flows more freely and the electrical current flow in the apparatus increases. Because of this increase in the current flow, the ion removal rate increases. Another advantage of such an apparatus is a decrease in power consumption because the applied voltage can be made smaller.
On the other hand, hardness components such as Ca and Mg, which originally exist in the concentrate water in small amounts, become increasingly concentrated as the concentrate water is circulated and reused, and, over time, more rapidly deposit in the concentrate chambers or in the electrode chambers to form scales. When scales are formed in the concentrating chambers or in the electrode chambers, the electrical resistance at the area where the scales are formed increases and less electric current flows at that section. For the same amount of current to flow as when no scales exist, the applied voltage must be increased and, therefore, power consumption must be increased. When still more scales are formed, the voltage must further be increased, eventually to the point where the applied voltage exceeds the maximum voltage of the device, at which point the current begins to decrease. In this case, sufficient current for ion removal cannot be applied, and the quality of the treated water deteriorates.
Some methods employed to prevent hardness components from concentrating in the concentrate water include (a) applying a softening treatment to the water to be treated by a reverse osmosis membrane apparatus which acts as a pretreatment apparatus for the EDI apparatus, (b) applying a softening treatment to the permeate water from the reverse osmosis membrane apparatus (the water to be treated by the EDI apparatus) which acts as a pretreatment apparatus for the EDI apparatus, and (c) increasing the amount of the concentrate water to be discarded from the EDI apparatus.
However, methods (a) and (b) above require providing a water softening apparatus and handling of regeneration chemicals and increase the cost and complexity of the facility. Method (c) above is undesirable in that the water usage rate (recovery rate) of the apparatus decreases when the concentration of hardness components in the water to be treated is relatively high.